1. Field of the Invention
The present invention is related to thermionic generators, and in particular to thermionic generators fabricated using micromachining methods.
2. Description of The Related Art
Most electricity is generated at a power station by a process in which heat is used to convert water to steam. The steam expands through a turbine device causing it to rotate. This powers a generator unit, which produces electricity. The heat is provided by burning a fuel such as coal, oil, gas, or wood, or from nuclear, solar or geothermal energy.
On a smaller scale, the generator unit may be powered by an internal combustion engine, such as a diesel or petrol driven motor. Similarly, the alternator used with the internal combustion engine in every type of automobile for providing electricity to the vehicle is powered by the rotating drive shaft of the engine.
All these devices use moving parts which are subject to friction and wear, and only a percentage of the heat generated is converted into electricity.
The thermionic generator, a device for converting heat energy to electrical energy, was first proposed by Schlieter in 1915. This device depends on emission of electrons from a heated cathode. In a thermionic generator, the electrons received at the anode flow back to the cathode through an external load, effectively converting the heat energy from the cathode into electrical energy at the anode. Voltages produced are low, but Hatsopoulos (U.S. Pat. No. 2,915,652), herein incorporated by reference, has described a means of amplifying this output.
One of the problems associated with the design of thermionic converters is the space-charge effect, which is caused by the electrons as they leave the cathode. The emitted electrons have a negative charge which deters the movement of other electrons towards the anode. Theoretically, the formation of the spacecharge potential barrier may be prevented in at least two ways: the spacing between the electrodes may be reduced to the order of microns, or positive ions may be introduced into the cloud of electrons in front of the cathode. Additionally, in practice, more difficulties remain, such as having low efficiency, costly to fabricate, and, particularly in the high-pressure ignited mode, do not have a long life.